Ensemble casts and Loads and Loads of Characters are two things that I used to like and may still do. On a whim, I did a barebones Google search for books with ensemble casts, and found that the vast majority of works in which they feature are more visual media than literature. Comics, obviously, and television (which executes the ensemble very differently from film). Goodreads made me sad, as the list I found there looked like 90% comics. I also found that there are a lot of readers who clearly do not know what an ensemble cast is, but I digress.
Possible reasons all immediately suggest themselves. Readers tend to be highly resistant to multiple points of view, especially as writers tend to have a very small repertoire of voices (very very often only one), and never seem to know how to format the changes. When any medium introduces characters in an obvious lineup or queue, I cringe. But in visual media, appearance can be used to distinguish each character instantly, without having to deal with any text at all. So you don’t have to juggle names right away. (it also means that juvenile character lineups ought to be destroyed and never done again) Books, even with illustrations, have to rely on the text, and dropping in too many names and terms at once rarely goes well.
They’re also written differently though. Television and comics are far more supportive of a group recognised as the protagonist (each individual can be well-developed, but still) where books revel in the Almighty I, the single hero whose friends are peripherals, and whose parents are generally killed off by laziness. I don’t wonder why some people read comics and not books. A lot of people who love to read still hate first person perspective. It may be this narrow milieu that puts them off.